Tap



y 1940 2 G. H. STIMSON 2,202,236

TAP

Filed April 27, 1938 Patented May 28, 1940 UNITED STATES 2,202,235 TAP.

Glen H. Stimson, Royal Oak, Mich., assignor to Greenfield Tap and Die Corporation, Greenfield, Mass., a corporation of Massachusetts Application April 27, 1938, Serial No. 204,489

4 Claims.

This invention relates to threading taps and particularly'to taps of the type generally known as gun taps in which the main flutes merge at the entering end with auxiliary or gun flutes extending at an angle. an angled series of cutting teeth which project the chips forwardly along the gun flute and ahead of the tap. A tap of this type is shown in the patent to Wells, No. 1,165,822.

The general purpose of the invention is to improve the chip cutting and discharging action of a gun tap. More specifically, the invention includes improvements in the contour and relative angles of attack of the cutting teeth, the contour and surface of the gun flute, and the provision of means and methods for producing the desired construction and arrangement.

Other objects and advantages of the invention, including the construction and relative arrangement of parts, will appear from the following description considered in connection with the accompanying drawing, in which,

Fig. 1 is a side View of a gun tap with parts broken away;

Fig. 2 is an end view from the left of Fig. 1 on an enlarged scale, showing the position of a grinding cylinder;

Fig. 3 is an enlarged fragmentary side view of the entering end of a gun tap indicating the position of the grinding cylinder and a grinding wheel;

Fig. 4 is a view similar to Fig. 2 indicating the position of a conical grinding wheel;

Fig. 5 is similar to Fig. 3, likewise showing the conical grinding wheel;

Fig 6 is a view similar to Fig. 2 showing a different arrangement; and

Fig. '7 is a transverse sectional view of the tap shown in Fig. 3 indicating the rake angle of th cutting teeth.

The gun tap I] includes the usual tap structure comprising shank l2 and lands l3 carrying leading threads 14 and cutting threads l5, which are shown as taper ground. Lands it are separated by the main longitudinal flutes [6 which are arcuate in cross section.

Each main flute l6 merges with a gun flute l1 extending downwardly from the main flute to the entering end of the tapwhen viewed as in Fig. 3. Each gun flute I1 extends across the entering ends of cutting threads l5, the angle between the surface of flute I6 and the radius from the axis of the tap to the apex of said end of a thread I 5 constituting the rake or angle of attack of said thread as illustrated at alpha and This arrangement provides beta in Figs. 2, 4 and 6. As said ends constitute the cutting instrumentalities of the tap they will be referred to herein for convenience as teeth I8.

With the arrangement shown in Figs. 2 and 3, the downward inclination of the flute I1 toward the tap end produces a progressively increasing width of the cylindrical portion of said flute, carrying teeth l8 progressively higher on the cylinder surface and therefore increasing correspondingly the rake of said teeth. As illustrated in Fig. 7, the rake angle of the cutting edge of the teeth I8 is the angle between the radius of the axis of the tap and the surface of the gun flute I? at the apex of the tooth. This is to a certain extent balanced by the progressively re-' duced diameter of threads I5 due to the taper grind, where the tap is of this type; but in general, and in the arrangement shown, the rake of teeth l8 will still be progressively greater toward the tap end. As illustrated in Fig. 2 the threads at the tap end have a rake alpha of substantially 35 while the threads at theroot of the taper. have a rake beta of about 27.

The gun flute H may be formed by a milling cutter ordisk grinding wheel having a cutting face that is semi-circular in cross section, traveling longitudinally of the tap in a suitable path, the path of the semi-circular edge portion of the forming device corresponding to the lower half of the above mentioned cylinder. tion of a properly positioned disk grinding wheel IQ of this type is indicated in Fig. 3.

This method of forming the gun flute tends to produce relatively inconspicuous but definite grooves and ridges extending longitudinally along the flute. While these grooves and ridges are so A marginal porslight as to be apparently inconsequential in the relatively slight grooves or ridges 20 extend in a generally transverse or circumferential direction in gun flutes [1, as indicated diagrammatically in Fig. 1. v

These results may conveniently be obtained I by means of a cylindrical polishing or abrading tool such as the cylindrical grinding wheel 2| indicated in Figs. 2 and 3, the diameter of which is suitably proportioned to the desired cylindrical surface of gun flute II; that is, if the cylindrical wheel 2| forms the final groove, it will have the same diameter as the cylinder of generation of the flute; whereas if it is intended primarily to surface or polish the flute, its diameter will be not greater than that of such cylinder. The use of the grinding wheel 21 will serve to destroy the longitudinal grooves and ridges in gun flute l1 and it may be constructed and operated to produce the circumferential grooves and ridges 20.

Therake of the cutting teeth i8 is of substantial importance, and seriously affects the efficiency and life of the tap. It has been found advantageous to shape the portion of the gun flute adjacent the teeth 18 to conform to the surface of a cylinder extending downwardly from the main flute 16 toward the end of the tap, and a gun flute ll of desirable contour for expelling the chips in characteristic manner is provided when such cylinder lies parallel to a plane through the tap axis intersecting the flute I6, and of such diameter that its outer face is not substantially beyond a plane tangent to threads I i and parallel to said axial plane. This arrangement is shown in Figs. 2 and 3. In this construction however, the rake of cutting tooth 18a which ordinarily makes the first cut in the work is substantially greater than that of the final cutting tooth it). The eiiiciency of the tap depends to a substantial extent upon the effective operation of tooth 18], whose apex is necessarily sharp. It is likewise important that the initial tooth I811 be given a proper rake, since its action particularly on relatively stringy metal is greatly enhanced thereby. A feature of the invention is to provide an arrangement in which both terminal teeth E80. and if may have the proper rake for their particular operations, the intermediate teeth l8 having a suitable intermediate rake. Another feature is the accomplishment of this object while retaining the desirable contour of the gun flute I'l suitable for eflicient expulsion of the chips. Specifically, all of the teeth 18 may be given the same rake without seriously distorting the desired gun flute contour.

This may be accomplished by employing in the gun flute a grinding or cutting device having an effective conical surface arranged with the smaller diameter portion operating on the final cutting tooth l8 and the larger diameter portion effective along the initial cutting tooth 58a. With this system the increased diameter adjacent the initial teeth may be designed to compensate for the longer are of contact already described in connection with Figs. 2 and 3, so that the desired widening of the gun flute I! toward the tap end will be retained and at the same time a substantially uniform rake of all of the cutting teeth 58 will be. produced. This embodiment is illustrated in Figs. 4 and 5, employing the conical grinding wheel 22.

This arrangement likewise may be employed to provide desired variations in rake between teeth Mia and i 8], by altering the taper of the grinding wheel or by varying the angular position of its surface at the intersection with the teeth I8.

This is illustrated, for instance, in Fig. 4 where the end threads have a rake alpha of about 10 while the root threads have a rake beta of about 20.

Variations in the rake of the cutting teeth within predetermined limits may be obtained by properly proportioning and positioning the cylindrical grinding wheel 2| relative to the cutting teeth [8' as indicated in Fig. 6. With this arrangement the wheel corresponding to wheel I9 is suitably located with its cylindrical surface in final grinding position at the desired rake angle in the zone of engagement with cutting teeth L5, I8a and I 8'), respectively. For instance, as illustrated in Fig. 6, the tip threads have a rake alpha of about 20 while the root threads have a similar rake beta of about 20. While this arrangement permits the maintenance of a nearly uniform rake or a predetermined variation in rake between the end teeth, it also imposes restrictions on the size of the grinding wheel 2| and the resulting proportions of the gun flute ll and in some cases the wheel 2| would be required to be shifted laterally during operation.

While a preferred arrangement has been illustrated, and has been shown substantially in correct relative proportions in the drawings, numerous modifications may be introduced without departing from the scope of the invention as set forth in the claims. For instance, while a tap with two flutes is shown in Figs. 1-6, any desired practicable number of flutes or the like may be employed. Likewise, wheels 2!, 2| and 22 are not restricted to surfacing or polishing, but may be utilized to form or shape the gun flute.

I claim:

1. A tap comprising a plurality of lands, flutes separating the lands including longitudinal flutes each merging into a divergent gun flute, threads on the lands forming cutting teeth at their intersections with the gun flutes, the surface of each gun flute forming circular arcs in planes normal to a common axis of generation, each passing through a cutting tooth, the angles between said arcs and the radii from the tap axis at the individual cutting teeth being substantially the same, providing a substantially uniform cutting rake. 2. A tap comprising a plurality of lands, flutes separating the lands including longitudinal flutes each merging into a divergent gun flute, and chamfered threads on the lands forming cutting teeth at their intersections with the gun flutes, the portion of each gun flute conforming to the surface of a cone at the cutting edges of said teeth so that the angle of rake of the end teeth is at least equalled by the angle of rake of the subsequent cutting teeth of said threads 3. A tap comprising a plurality of lands, flutes separating the lands including longitudinal flutes each merging into a divergent gun flute, and threads on the lands forming cutting teeth at their intersections with the gun flutes, the surface of each gun flute forming circular arcs in planes normal to a common axis of generation, each passing through a cutting tooth, the radius of said arcs decreasing progressively from the initial cutting tooth to the final cutting tooth, the angles between the arcs and the radii from the tap axis to the apices of the cutting teeth being substantially the same, providing a substantially uniform cutting rake.

4. A tap comprising a plurality of lands, flutes separating the lands including longitudinal flutes each merging into a divergent gun flute, and threads on the lands forming cutting teeth at their intersections with the gun flutes, the rake of the teeth being substantially uniform, the teeth merging into a portion of the gun flute arcuate in cross section and provided with transverse ridges of that type produced by transverse abraslon.

GLEN H..STIMSON. 

